The present invention relates to a spark timing control device for an internal combustion engine, and more particularly, to improvements thereto.
In order to make the actual spark timing of an internal combustion engine correspond as close as possible to the required characteristic thereof, which changes as a function of engine operating conditions, electronic spark timing control devices using a microcomputer have been developed to replace mechanical spark advance angle control devices.
One such electronic control devices uses time-chopping spark timing control. In this device, optimal spark timing advance angles depending on various instantaneous operational states of the engine, such as, for example, various engine speeds and intake vacuum pressure, are stored in advance in a memory table (ROM). In this case, each spark timing advance angle value is actually stored as a number of clock pulses representing the spark preparation time duration starting when a crankshaft is at a standard reference angle position and ending at the time of spark occurrence. A reference angle sensor derives a reference angle pulse whenever the crankshaft turns through a certain angle, for example, in the case of a 6-cylinder engine, 120.degree., corresponding to the spark timing difference between any two adjacent cylinders. Each time this reference angle pulse is produced, an optimal spark preparation time duration representative value corresponding to the intake vacuum pressure and the rotational speed (determined from the reference angle pulses) at that time is read out of the memory table. A spark signal is produced when a time duration representative value read from the memory table is reached by the time duration value; the time duration starts when the reference angle pulse is produced.
In this prior art device, a fixed-frequency clock pulse generator produces clock pulses which are counted from the moment when the reference angle pulse is produced. The spark is produced when the number of counted clock pulses coincides with the time duration representing value read out of the table memory.
As the engine speed decreases, the time required for the crankshaft to turn from the reference angle pulse position to the desired spark angular position increases so that a relatively large number of clock pulses occur as the shaft turns between the two positions. Usually, the frequency of the clock pulses is set relatively high, so that sufficient accuracy is obtained for relatively high crankshaft speed. Thus there is a need to count a relatively very large number of pulses in the relatively low rotational-speed region, thereby requiring a large capacity counter. Further, the processing tends to become complicated since such a large number of pulses must be handled.